Abstract
The Aksu River is one of the main rivers of Zhetysu (Seven Rivers) and flows into Lake Balkhash. The current ichthyofauna of the Aksu River is represented by seven indigenous and eleven alien species. The largest number of alien fish species are noted on a large part of the piedmont and estuary river sections. The structure and species composition of river fish populations have changed over the last 20 years. The Chinese false gudgeon Abbottina rivularis and the Siberian dace Leuciscus baicalensis have appeared in it, and the former has reached a substantial abundance. The species structure of fish populations in the piedmont part is extremely unstable and depends on its hydrological regime.